There exist numerous methods intended to lower the consumption of a stepping motor by adjusting the quantity of electrical energy furnished to the motor during a driving pulse to the resistant torque which the rotor must overcome. Since such torque cannot be directly measured, these methods in fact adjust such quantity of electrical energy as a function of the measurement or the calculation of a value of a physical characteristic, the variation of which depends more or less directly from said torque as a function of time.
The determination of the resistant torque applied to the rotor during a driving pulse may be made following the end of said pulse as for instance in the U.S. Pat. No. 4,212,156. In this patent the variation of current induced in the motor winding by the oscillations of the rotor at the end of the driving pulse is taken as a measurement of this torque and the duration of the following driving pulses is modified, if necessary, as a function of the result of such measurement.
The determination of the resistant torque applied to the rotor may likewise be made during each driving pulse as for instance in the European patent application EP-A-0.060.806. In this application, it is the rate of variation of the voltage induced in the motor winding through the rotation of the rotor which is taken as a measurement of the torque.
These methods as well as numerous others not described herein all present the difficulty that the physical characteristic which they employ as a measurement of the resistant torque, is not truly representative of such torque.
There results therefrom that the control of a stepping motor according to any of these known methods cannot in a practical sense be optimum. This means that if the arrangement applying the chosen method is arranged in a manner such that the motor operates correctly in all possible situations, the consumption of such motor will generally be clearly greater than its theoretic minimum consumption. Should one attempt to modify the characteristics of the control circuit in a manner such that the consumption of the motor diminishes so as to approach its theoretical minimum, then the reliability of operation of such motor diminishes, i.e. its rotor no longer turns correctly responsive to each driving pulse.
Sometimes it is necessary to apply to a stepping motor a driving pulse such as to bring about with certainty the rotation of the rotor, even if the resistant couple applied to the latter is at its maximum value.
This case is shown in particular when the motor is controlled according to a method such as that described in the U.S. Pat. No. 4,272,837 for example.
Such a method consists in particular in applying to the motor a long duration pulse referred to as a catch-up pulse when a suitable circuit has detected that the rotor has not turned in response to a normal driving pulse of short duration.
The duration of the catch-up pulse is evidently determined in a manner such that it brings about rotation of the rotor even if the resistant torque applied thereto is at its maximum value.
However, it may happen that the case of failure by the rotor to rotate in response to a normal driving pulse is only momentary and that the torque applied to the rotor during the following catch-up pulse is small. In such a case the quantity of electrical energy provided to the motor during this catch-up pulse is much too high and it is possible that the rotor of the motor makes several steps instead of one only in response to such catch-up pulse.
The detection of the rotation or non-rotation of the rotor which is necessary in the control methods which have just been mentioned may be accomplished in various manners.
Thus, for instance, in the method described in the U.S. Pat. No. 4,272,837 mentioned hereinabove this detection is obtained by applying to the stepping motor a detection pulse of very short duration a certain time following the end of each driving pulse. The amplitude of current which circulates in the motor winding at the end of this detection pulse enables one to determine whether the rotor has turned or not in response to the preceding driving pulse.
The difference between the currents circulating in the winding in the one and the other case, rotation or nonrotation of the rotor, is however small, thus rendering difficult certain detection of non-rotation. Furthermore, the current measurement may be distorted if the rotor is moving when the detection pulse is applied to the motor, either because the rotor has not yet finished oscillating about its equilibrium position or, because it has been put into motion for instance by a shock.
There exist other methods enabling detection of whether the rotor of the stepping motor has turned or not in response to a driving pulse. These methods will not be described here otherwise than to point out that generally they present the same difficulties as the method described hereinabove.
The purpose of the present invention is to provide a control method for a stepping motor which avoids the difficulties of the methods described hereinabove and which enables according to the manner in which it is applied, to reduce the consumption of the stepping motor almost to its absolute minimum or to cause the rotor of the motor to turn through one and only one step with a large measure of reliability and this whatever may be the resistant torque applied to this rotor or to enable certain detection of rotation or non-rotation of the rotor in response to a driving pulse.
A further purpose of this invention is to provide an arrangement for applying this method.